It is known an information recording medium comprising a substrate and a recording layer capable of writing or reading out information by focusing light such as laser beam onto the recording layer. It is also known information recording medium having laminated two disc substrated by an adhesive, each disc substrates comprising a substrate and a recording layer, so that the recording layers are disposed opposite to each other.
Adhesives used for laminating two disc substrates include a UV-curing adhesive, a double coated adhesive tape, a cold setting adhesive and a hot melt adhesive. Generally speaking, two disc information recording media laminated with a UV-curing adhesive or those laminated with a double coated adhesive tape have such problems that they receive serious deformation such as warpage, cracks and corrosion holes, and moreover, are poor in adhesive strength. Furthermore, a two disc information recording medium laminated with a double coated adhesive tape has such a problem that air comes in between the laminated surfaces formed by the tape and the recording medium layers of the discs of the medium. On the contrary, a two disc recording medium laminated with a cold setting adhesive is excellent in appearance and in adhesive strength, but has such a drawback that the medium itself receives deformation such as warpage.
On the other hand, a two disc information recording medium laminated with a hot melt adhesive applied on the surface disc substrates by way of a roll coater, etc. is excellent in appearance and adhesive strength, and moreover, deformation such as warpage received in the medium is much smaller as compared with a two disc information recording medium laminated with the aforementioned adhesives other than the hot melt adhesive. Therefore, hot melt adhesives have attracted much attention since they are expected to be quite promising in the future. However, a two disc information recording medium laminated with the hot melt adhesive has such problems that when the recording medium is kept in high temperature and high humidity conditions (for example, in conditions where the temperature is 70.degree. C. and the humidity is 80%) for a long time (for example, 300 hours), deviation takes place between the two laminated disc substrates or the deformation of the medium such as warpage tends to be serious.